1. Field of the Invention
The present invention relates to a rotating device which is used in devices that perform precision measurement of measured objects or precision processing of workpieces, and, more particularly, to the frictional resistance between the fixed portion and the movable portion, the stopping of the driving of the movable portion, and a fine positioning mechanism and fine positioning method of a rotor which are used for performing adjustment during attachment of a rotor, such as an encoder disk, to a rotary shaft so as to eliminate eccentricity and so on of the rotor with respect to the rotary shaft.
2. Description of the Related Art
In the prior art, a wide range of various types of rotary tables were used mounted onto machine tools such as NC lathes and milling machines or precision measuring machines such as roundness measuring instruments and three-dimensional measuring instruments.
Such types of rotary tables, for example, contain a fixed portion and a movable portion, with a turntable, on which the processed workpiece or measured object is placed, mounted on the movable portion. This turntable is provided with an integral shaft and comprises the rotary table by means of this shaft and the shaft of a servo motor being coupled via a speed reducer or coupling, etc.
However, as the prior art typically consisted of the servo motor, coupling and turntable oriented in a row in the axial direction, the height of such a rotary table was considerably high. As a result, when such rotary tables are attempted to be applied, for example, in three-dimensional measuring instruments for which the measuring range in the vertical direction is set in advance, the actual measuring range is reduced making such rotary tables undesirable for use.
As such, rotary tables were conceived in which the motor is oriented not in the axial direction, but rather in the horizontal direction thereby reducing the total height of the rotary table. Examples of these are indicated in FIGS. 13, 14 and 15.
In the block diagrams of FIGS. 13 and 14, rotary table 200 contains fixed portion 201, consisting of a casing and so on, and movable portion 202 which is allowed to rotate freely supported on said fixed portion 201, and turntable 203 is contained within said movable portion 202. Turntable 203 is provided with a shaft not shown in the drawing. A cross roller bearing (not shown) is mounted on this shaft, or in other words, between movable portion 202 and fixed portion 201. Moreover, drive device 210 is coupled to this shaft.
As is indicated in FIG. 15, drive device 210 contains worm wheel 211 attached to the previously mentioned shaft (not shown), and worm gear 212 engages with said worm wheel 211. In addition to said worm gear 212 being supported by fixed portion 201 via two bearings 213, speed reducer 217, comprised of primary gear 214, secondary gear 215 and tertiary gear 216, is mounted on one end of said worm gear 212, with output shaft 219 of servo motor 218 coupled to said tertiary gear 216.
In rotary table 200 which is composed in this manner, servo motor 218 must first be driven in order to rotate turntable 203. The torque of said servo motor 218 is transmitted to worm gear 212 after being reduced by speed reducer 217. Moreover, after the direction of the rotary shaft of said servo motor 218 is changed by worm gear 212 and worm wheel 211, the torque of said servo motor 218 is transmitted to turntable 213 through a shaft (not shown) formed in an integrated manner on worm wheel 211, resulting in rotation of turntable 213.
Thus, in the case of rotary table 200, turntable 203 is driven indirectly through speed reducer 217, worm gear 212, and worm wheel 211 by servo motor 218.
However, in such a case, due to turntable 203 being driven through worm wheel 211, worm gear 212 and speed reducer 217, the structure of the device becomes quite large. Moreover, when stopping and rotating is repeated, unevenness forms in the rotation of turntable 203, especially due to the action of backlash that occurs at this time, resulting in a decrease in rotational accuracy.
As a result, this exposes the fault of difficulty in the precision measurement of measured objects and in the precision processing of processed workpieces.
In addition, when this rotary table 200 is driven for a long period of time, the temperature in the vicinity of servo motor 218 rises locally due to generation of heat within said servo motor 218. This then leads to the occurrence of strain in fixed portion 201 and eventually in turntable 203. Thus, there is the risk of decreased rotational accuracy in terms of this point also.
Furthermore, in this rotary table 200, there are many cases in which a contact-type, cross roller bearing not shown in the drawing is externally mounted on the shaft (not shown) which is formed in an integrated manner with turntable 203.
Such contact-type bearings having a rotational accuracy of 1 micrometer or less are extremely rare. If bearings having a low level of accuracy are used, the shaft (not shown) of turntable 203 will bend which will result in the risk of decreased rotational accuracy. In addition, the use of specially selected high-precision bearings will not only result in a remarkable decrease in productivity, but will also result in the disadvantage of raising the cost of said turntable 200. As such, the first objective of the present invention is to provide a rotary table to be used as a rotating device which is compact, is minimally subjected to the effects of backlash, and does not produce unevenness in rotation.
In addition, as is indicated in FIG. 16, bearing 204 and O-ring 205, which prevents the entry of oil, etc. into the inside of the device, are mounted between turntable 203 and fixed portion 201. Moreover, highly rigid projection 206 is provided beneath bearing 204 projecting outwardly in the radial direction in an integrated manner from turntable 203.
Furthermore, clamping device 230, which clamps turntable 203 to fixed portion 201, is mounted at the bottom of fixed portion 201.
Clamping device 230 contains cylinder chamber 231 formed in fixed portion 201, and said cylinder chamber 231 is provided with air connection hole 232 which supplies air to said cylinder 231. In addition, cylinder cap 234 is fit into said cylinder chamber 231 via O-ring 233. Moreover, piston rod 237 is inserted via rod packing 236 into center hole 235 formed in said cylinder cap 234.
Shaft 241 is provided in an integrated manner on the top of piston rod 237 via piston 239 having piston packing 238 around its perimeter. The top of said shaft 241 projects from fixed portion 201 and opposes projection 206 of turntable 203.
On the other hand, together with coil spring 242 for applying pushing pressure being mounted on the outside of the bottom of shaft 241, said coil spring 242 is also mounted between spring holder 243 provided on fixed portion 201 and piston 239. Thus, although piston 239 is forced in the downward direction in the drawing by the action of coil spring 242, shaft 242 is constantly separated from projection 206 of turntable 203. On the other hand, when air under high pressure is supplied to cylinder chamber 231, piston 239, and in turn, shaft 241 move upward in opposition to coil spring 242.
Furthermore, through hole 244 is formed in shaft 241, piston 239 and piston rod 237 to connect the inside of the chamber in which coil spring 242 is mounted with the outside.
When an emergency stop button, etc. not shown in the drawing is pressed in order to instantly stop turntable 203 with clamping device 203 during rotation of turntable 203, high-pressure air is first supplied to cylinder chamber 231 via air connection hole 232 from a compressed air source (not shown). As a result, since the pressure inside cylinder chamber 231 rises causing piston 239 and shaft 241 to be displaced in the direction of Z indicated with an arrow in opposition to the pushing pressure of coil spring 242, the end of shaft 242 makes contact with projection 206 causing projection 206 to be pushed towards fixed portion 201 thereby stopping rotation.
However, in this case, as projection 206 is of a comparatively rigid structure and is formed in an integrated manner from turntable 203, projection 206 is naturally subjected to force in the Z direction, or in other words, in the thrust direction with respect to the rotary shaft of turntable 203, due to the pushing pressure in the Z direction by shaft 241. This force ends up being transmitted to turntable 203.
Moreover, in the case shaft 241 pushes on projection 206 at a slight angle in the radial direction with respect to the vertical direction due to the effects of incorporation tolerance and so on, force in the thrust direction (indicated with a Z) along with force in the radial direction (indicated with an X), that is perpendicular to that, are generated during clamping. These forces being applied in two directions are then transmitted to turntable 203 via projection 206.
From among the forces that act on turntable 203, force in the thrust direction is transmitted to bearing 204 that is provided between turntable 203 and fixed portion 201. This results in the contact position of bearing 204 and turntable 203 being shifted slightly out of position which results in the formation of a gradient with respect to the horizontal direction of turntable 203. On the other hand, the previously mentioned force that is applied in the radial direction (indicated with an X) acts essentially acts in the radial direction of turntable 203. This force that acts in the radial direction ends up creating eccentricity in turntable 203, and deteriorates the rotational accuracy combining with the positional displacement caused by the force in the thrust direction.
This results in the disadvantage of it becoming difficult to perform precision measurement or precision processing of measured objects and workpieces attached to turntable 203.
Thus, the second objective of the present invention is to provide a rotary table to be used as a rotating device in which there is extremely little creation of eccentricity in the turntable and virtually no deterioration of accuracy.
Moreover, at the time of incorporation of a rotary encoder into the rotary table, the rotary shaft of the rotary table and the encoder disk, functioning as the rotor which is a portion of the rotary encoder, may become slightly off center by several micrometers.
When the rotary table is allowed to rotate under such circumstances, the angular indexing accuracy of said rotary table is lowered making precision measurement of measured objects and precision processing of workpieces difficult.
As such, various devices have been proposed which compensate for the eccentricity at a fixed position with respect to the rotary shaft of the encoder disk acting as a rotor.
An example of one of these devices involves the performing of fine positioning by subjecting the rotary encoder to slight vibrations. However, in the case of such devices, since a vibrating device such as a vibrator is used, in addition to the size of the device increasing, it also has the disadvantage of resulting in higher costs.
In addition, set screws, with which the rotor is attached to the rotary shaft, are provided equidistant to each other at 90 or 60 degree angles. Positioning is thus performed by changing the amount by which these set screws are pushed in. However, in the case of such a mechanism, since the amount the set screws are pushed in is determined according to the product of the lead of the set screws and the angle of rotation, the lead of the set screws should be made quite small in order to allow the making of fine adjustments by reducing the amount the set screws are pushed in with respect to the same angle of rotation. However, as there are limits as to how small the lead of the set screws can be made, it was difficult to allow the making of adequate fine adjustments such as that in the case of positioning on the order of a sub-micron meter.
Thus, the third objective of the present invention is to provide a fine positioning mechanism and fine positioning method that allows fine positioning of a rotor, such as an encoding disk, attached to the rotary shaft of a rotating device to be performed easily while also preventing the occurrence of eccentricity.